Absorption of biomolecules, such as amino acids and proteins, is critical to cellular function. About 75 percent of the solids in the mammalian body are proteins, including enzymes, polypeptides such as cytokines, nucleoproteins, transport proteins, and structural proteins. The principal functional constituents of these proteins, amino acids, polypeptides and isolated amino acids, are also important for cellular metabolic functions. The amino acid glutamine, for example, serves important functions in metabolism, including transport of carbon and nitrogen between tissues. It is a precursor for hepatic and renal gluconeogenesis, as well as urea synthesis in the liver and ammonia production in the kidney. A number of cell types, particularly the cells of the intestinal mucosa, also utilize large amounts of glutamine as their major source of respiratory fuel.
The effectiveness of amino acid supplementation for treatment of a variety of physiologic disorders has been demonstrated. D-serine supplementation, for example, augments the beneficial effects of antipsychotics for the treatment of schizophrenia. (Tsai, G., et al., Biol. Psychiatry (1998) 44(11): 1081–1089.) L-tryptophan or 5-hydroxytryptophan supplementation has been shown to improve symptoms of depression, anxiety, insomnia and pain in patients with fibromyalgia. (Juhl, J. H., Altern. Med. Rev. (1998) 3(5): 367–375.) Dietary supplementation with 8 essential and 9 nonessential amino acids provided improved health, tone, and mood in dialysis patients, in whom protein malnutrition is a common problem. (Mastroiacovo, P., et al., Clin. Ther. (1993) 15(4): 698–704.) Nutritional supplementation with aspartic acid has been suggested for the treatment of Canavan disease, a rare recessive autosomal genetic disorder generally resulting in death within several years of onset. (Baslow, M. H., et al., J. Mol. Neurosci. (1997) 9(2): 109–125.) L-lysine has also been demonstrated to have therapeutic use for lesions associated with herpes simplex virus type 1 (HSV-1). (Ayala, E. And D. Krokorian, J. Med. Virol. (1989) 28(1): 16–20.)
Glutamine supplementation has been shown to provide numerous benefits, including stimulation of certain cells of the immune system and general promotion of cellular growth. Depletion of glutamine results in atrophy of epithelial tissue, with associated bacterial translocation. Clinical supplementation of glutamine reduces epithelial atrophy and accelerates recovery.
Dietary glutamine supplementation has been proposed for the treatment of patients recovering from surgery or suffering from sepsis, inflammation, burns, or trauma. Topical administration, usually in the form of a “swish and swallow” solution for oral use to repair the damaged epithelial tissue of mouth or esophageal sores, can be effective in many patients who have undergone bone marrow transplantation or chemotherapy. (Skubitz, et al., J. Lab. Clin. Med. (1996) 127(2): 223–8; Anderson, et al., Bone Marrow Transplant (1998) 22(4): 339–44.) Formulations for the administration of amino acids, particularly glutamine, are described in U.S. provisional patent application No. 60/134,442 filed May 17, 1999 and incorporated by reference herein.
The effectiveness of amino acid supplementation has been limited in some individuals due to aging or disease. Effective supplementation with certain amino acids is further limited to varying degrees by the low aqueous solubility and limited cellular uptake of some amino acids. Glutamine, for example, exhibits a low solubility in water (48 g/l at 30° C., 26 g/l at 18° C., 18 g/l at 0° C.; The Merck Index, 12th Edition) and a low chemical stability in aqueous solution (11 days at 22–24° C.). (Cardona, P., Nutr. Hosp. (1998) 13(1): 8–20.).
Transport of small molecules into various cell types is controlled by alternate transport systems, making it more difficult to devise methods for increasing cellular uptake into particular cell types. Despite the need for methods to enhance the uptake of amino acids and other small molecules, methods for increasing initial direct absorption of amino acids, peptides and other compounds into cells such as epithelial cells, the type of cells initially responsible for initial uptake of many bioactive compounds, has not been described.
Therefore, a continuing need exists for methods to increase cellular uptake of bioactive compounds into mammalian cells.